Inclusions of carbon in ingots of silicon carbide grown by the modified Lely method
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Inclusions of Carbon in Ingots of Silicon Carbide Grown by the Modified Lely Method D. D. Avrova, S. I. Dorozhkina, Yu. M. Tairova, A. Yu. Fadeeva^, and A. O. Lebedevb aSt.
Petersburg State Electrotechnical University, St. Petersburg, 197376 Russia ^e-mail: [email protected] bIoffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia Submitted February 1, 2008
Abstract—The problem of the appearance of carbon inclusions in single-crystal silicon carbide ingots grown by the modified Lely method (the so-called graphitization of the ingot) is analyzed. It is shown that the process of graphitization of the ingot is not related to a deficit of silicon in the growth cell; in contrast, it is excess of silicon at the growth surface that inhibits the ingot growth rate and gives rise to intense corrosion of the graphite fittings. PACS numbers: 61.72.Dd, 81.05.Hd, 81.10.Bk DOI: 10.1134/S1063782608130034
1. INTRODUCTION A number of publications [1–3] have been concerned with an analysis of factors that give rise to appearance of carbon inclusions in a SiC single-crystal ingot (the so-called graphitization of the ingot) in the course of its growth by the modified Lely method. For example, Li et al. [1] noted that it is important to avoid appearance of carbon inclusions in the seed (graphitization of the seed) before the onset of growth. Intentional thermal etching of the seed suggested by Anikin et al. [4] leads to graphitization of the seed with subsequent formation of defects in the growing crystal. Exhaustion (with respect to silicon) of the source as a result of escape of silicon from the cell and active interaction of silicon with graphite fittings is considered as the main cause of appearance of carbon in the growing ingot [2, 3]. Thus, maintenance of high pressure of silicon at the seed surface is thought to be a topical problem [1]. In order to compensate for losses of silicon, it is suggested to use either an additional silicon source [5] or nonporous tantalum crucibles [6] that prevent the escape of silicon from the growth cell. At the same time, it is noted that, in a number of cases, an excess of silicon brings about appearance of silicon drops on the growth surface and also a change in the growing polytype of silicon carbide [5–7]. In this study, we consider sequentially the possible factors that lead to appearance of carbon inclusions in the growing ingot. 2. EXPERIMENTAL We grew the bulk SiC single crystals of the 4H and 6H polytypes using the modified Lely method in a
resistance-heated furnace at temperatures of 2000– 2200°C using a growth cell made of low-porosity graphite (MG-1, produced in Russia). The duration of experimental cycles ranged from 5 to 50 h. The argon pressure pAr was in the range from 5 to 100 Torr. The seed holder and the seed itself were mounted at the top cover of a standard growth cell; a powder-like source of silicon carbide was located at the bottom of the cell. This source co
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